What are the diff between tow bar, dolly, trailer?

If you tow 4 down then you have to modify the toad for a tow bar, lighting and really should include a braking system. If you use a dolly then all you can use it for is to tow a vehicle. You cannot back up 4 down or dolly. The best is really a trailer with wheel sensors. Just put a hitch on the toad so you can use it to spot the trailer when it's unloaded. The additional weight only counts when climbing hills.